As this kind of premixing apparatus, the following is known in JP-A-2015-230113; that is, a downstream end of that gas supply passage for supplying fuel gas which has interposed therein a flow control valve, is connected to a gas suction section disposed in an air supply passage on an upstream side of the fan. The premixing apparatus comprises: an air resistance changeover means for changing over, between high and low, a ventilation resistance in that section of the air supply passage which is on an upstream side of the gas suction section; and a gas resistance changeover means for changing over, between high and low, a ventilation resistance in that section of the gas supply passage which is on the downstream side of the flow control valve.
By the way, in case a proportional valve is used as the flow control valve, the proportional valve is controlled so that the fuel gas can be supplied in amount depending on the required combustion amount. Further, the fan revolution is controlled depending on the required combustion amount so that the air-fuel ratio of the air-fuel mixture to be supplied to the burner becomes constant. However, in case the required combustion amount falls below a predetermined value and, as a result, the fan revolution has fallen below a lower limit revolution below which the proportional characteristics of the air supply volume cannot be maintained, or in case the electric current to the proportional valve (electric current to be charged to the proportional valve) has fallen below a lower limit electric current below which the proportional characteristics of the gas supply amount cannot be maintained, the air or fuel gas in amount depending on the required combustion amount can no longer be supplied.
In addition, as the flow control valve, there is a case in which is used a zero governor that maintains the secondary gas pressure to the atmospheric pressure. In this case, the amount of fuel gas supply varies with the differential pressure between the atmospheric pressure that is the secondary gas pressure and the negative pressure inside the air supply passage. And since the negative pressure inside the air supply passage varies with the fan revolution, the amount of fuel gas supply varies with the fan revolution, i.e., the amount of air supply. It follows that, by controlling the fan revolution depending on the required combustion amount, the amount of air and fuel gas depending on the required combustion amount will be supplied to the burner.
Also in this arrangement, if the fan revolution has fallen below a lower limit revolution at which the proportional characteristics of the air supply amount can be maintained, the air or fuel gas depending on the required fuel amount can no longer be supplied. Therefore, when the required combustion amount has fallen below a predetermined amount, it is necessary to increase the ventilation resistance in the air supply passage by the air resistance changeover means. Then, without making the fan revolution below the above-mentioned lower limit value, the amount of air depending on the required combustion amount below the predetermined value can be supplied. Further, only by increasing the ventilation resistance in the air supply passage, the amount of fuel gas supply will exceed the amount corresponding to the required combustion amount due to an increase in the negative pressure in the air supply passage. It is therefore necessary also to increase the ventilation resistance in the gas supply passage at the time when the ventilation resistance in the air supply passage is increased.
As a solution, in the above-mentioned prior art example, the following arrangement is employed; that is, when the required combustion amount has fallen below the predetermined value, the ventilation resistance in the air supply passage is increased by the air flow resistance changeover means, and also the ventilation resistance is increased by the gas flow resistance changeover means so that the amount of air and fuel gas can be supplied depending on the required combustion amount below the predetermined value.
In the above-mentioned prior art described in JP-A-2015-230113, the air resistance changeover means is constituted by a butterfly valve that is rotatably disposed in a section of the air supply passage that is on an upstream side of the gas suction section. In this arrangement, however, when the butterfly valve is rotated into a closed posture at right angles to the longitudinal direction of the air supply passage, the flow passage in the air supply passage will suddenly be throttled into an annular clearance between the inner circumferential wall surface of the air supply passage and the outer peripheral edge of the butterfly valve. As a result, there will be generated turbulence in the air flow, whereby wind noise is generated at the annular clearance, giving rise to noises.